Hermetic sealing of electronic devices is important in many areas of the electronics industry to insure an airtight environment for the various components of the device. Many devices (e.g., automotive airbag inflators) require high reliability for long periods of time and are typically hermetically sealed within a hollow, outer body. Electrical interconnection between the sealed electronic device and external circuitry is provided by conductive pins passed through apertures in the outer body. To provide a hermetic and insulative seal between the conductive pins and the outer body, the gap therebetween can be filled with glass to form a glass-to-metal seal.
There are two types of glass-to-metal seals. When hermeticity is predominately achieved by molecular bonding between the glass and metal components, it is referred to as a matched seal. To obtain molecular bonding, the surfaces of the metal components (e.g., the conductive pin and the outer body) are typically oxidized to provide a surface to which the glass can readily bond. Glass is disposed into the gap between the oxidized outer body and the oxidized conductive pin. For example, a glass preform may be inserted into the outer body and the pin may be inserted into a bore in the glass preform. Subsequent heating of the assembly above the softening point of the glass results in the glass flowing over the oxide coating of the metal components. As the assembly is cooled, a molecular bond is formed between the oxide surfaces and the glass. To avoid residual thermal stresses between the various components, the components typically have substantially matching coefficients of thermal expansion (CTEs).
Compression seals, on the other hand, achieve hermeticity by the interaction of residual stresses created between the various components due to differing CTEs. That is, the outer body typically has a CTE which is greater than the CTE for the glass preform. In addition, the glass preform may have a CTE greater than that for the conductive pin. To form the seal, the assembled components are heated to a temperature above the softening point of the glass preform, typically about 950.degree. C. or higher, so that the glass flows to fill the void between the conductive pin and the outer body. Upon cooling, the outer body contracts at a faster rate than the glass, thereby applying compressive forces on the glass and the conductive pin. These compressive forces provide the seal between the various components. An example of a compression seal is disclosed in U.S. Pat. No. 5,243,492, by Marquit et al., which is incorporated herein by reference in its entirety.
The conductive pins used in these applications are typically fabricated from iron-nickel alloys. In many applications, it is desirable to provide the exterior surface of the conductive pin with a corrosion-resistant coating, such as a noble metal (e.g., gold, silver, platinum, palladium, etc.). The most commonly used of the noble metal coatings is gold. In order to provide proper adhesion of a gold coating to the underlying metal, an intermediate coating of another metal, such as nickel, is typically provided. However, gold tends to substantially diffuse into nickel at temperatures of about 700.degree. C. and higher, thereby degrading the corrosion resistance of the coating. In addition, gold coating of the entire surface of the conductive pins prior to heating the assembly to form a matched seal is not practical because an oxidized coating cannot easily be adhered to a gold coating.
U.S. Pat. No. 4,706,382 by Suppinger et al. discloses that, in matched seals, plating conductive pins with gold prior to their assembly in a header is not a practical alternative due to the necessity of a preoxidized, plating-free metal surface to achieve an effective glass-to-metal seal. Plating prior to assembly is also not practical in the process disclosed by Suppinger et al. due to the sealing temperature of the glass utilized, i.e., about 950.degree. C. to 1,000.degree. C.
U.S. Pat. Nos. 4,788,382 to Ahearn et al. and 5,157,831 to Wang et al. both disclose that conductive pins can be plated with a layer of gold to improve corrosion resistance. Although it is unclear from the disclosure of these patents when the gold is plated onto the pins, the plating is believed to occur after formation of the hermetic seal because of the sealing temperatures utilized in these patents. For example, Ahearn et al. disclose the use of Corning 9010 or 9013 glasses, available from the Corning Corporation, which have sealing temperatures of about 1000.degree. C. Wang et al. disclose heating the glass to a temperature of between 750.degree. C. and 1500.degree. C. and also disclose that the metallic shell and metallic pin are heated in an oxidized atmosphere to form an oxide layer.
When a gold plated pin is desired, the gold is typically plated onto the conductive pin after assembly and formation of the hermetic seal by masking and/or selectively plating the conductive pins so that the gold does not plate onto the other metallic components of the electronic device. The masking and selective plating process is time-consuming and labor-intensive and therefore adds significantly to the overall cost of the device. In addition, the masking process is sometimes imprecise, resulting in gold inadvertently plating onto other areas and causing an electrical short or resulting in incomplete coverage of the entire exposed surface of the conductive pin leaving areas prone to corrosion.
Accordingly, there is a need for a relatively inexpensive and efficient process for forming a glass-to-metal seal with a coated conductive pin. It would be advantageous if such a process avoided the use of expensive and time-consuming masking and/or selective plating steps and avoided the possibility of coating the outer body and producing an electrical short. It would also be advantageous if the conductive pin could be reliably coated with a noble metal coating over substantially the entire exposed surface of the pin to reduce the likelihood of failure of the device due to corrosion at exposed, uncoated areas of the pin.